The invention described and claimed herein relates generally to the field of communications, and is particularly concerned with a radio communication system in which signals are exchanged between ground-based transmitters and receivers through one or more relay satellites.
Many radio communication systems utilize satellites, particularly geostationary satellites, as signal relays in order to cover large geographic areas and to eliminate interference from terrestrial sources In fixed satellite communication systems, earth stations at fixed geographic locations transmit radio signals to a satellite which receives, amplifies, and rebroadcasts the transmissions at a shifted frequency in order to avoid interference with the received signals. The retransmitted signals are received by other earth stations at fixed locations. In this manner, point-to-point communication links can be established. Mobile satellite communication systems operate in much the same manner, although in this case the signals are relayed between mobile terminals which are carried by automobiles, trucks, airplanes, ships, or other movable platforms.
Both fixed and mobile satellite communication systems share certain limitations. One limitation relates to the fact that the frequency generators or oscillators used on board the satellites in order to achieve the desired frequency shift are subject to frequency drift. The occurrence of such drift can result in distortion of the received signals or, in some cases, in the total loss of the relayed transmissions.
Once a satellite is placed into orbit, it becomes difficult or impossible to control any drift that may occur in the satellite oscillator. While it is possible in principle to provide onboard systems that eliminate or compensate for drift, this option is not available for satellites already in use. In any event, such an approach may be impractical in many instances due to satellite weight budgets and other factors.
Satellite oscillator drift poses a particular problem in satellite communication systems which employ spread spectrum coding of the transmitted signals. The phrase "spread spectrum" generally refers to methods of radio transmission in which the frequency bandwidth of the transmissions greatly exceeds the minimum necessary to communicate the desired information. Several types of spread spectrum coding are possible. So-called "direct sequence" systems are those in which the carrier frequency is modulated by a digital code sequence whose bit or "chip" rate is much higher than the information bit rate. Other types of spread spectrum systems include frequency hopping systems, in which the carrier frequency is switched among a plurality of predetermined values, and chirp modulation systems, in which the carrier frequency is swept over a wide band during a given pulse interval. The advantages of spread spectrum coding include improved noise immunity and the ability to allow use of the same frequency band by multiple users without mutual interference.
In all types of spread spectrum communication systems, it is necessary for the receiver to acquire (i.e., synchronize to) the transmitted signal before the data can be decoded. This process, which is ordinarily carried out by phase locked loop circuits, introduces a finite delay between initial reception of a spread spectrum transmission and recognition of the transmitted data. In the case of continuous mode spread spectrum transmissions, the acquisition or lockup delay occupies only a small part of the time domain of the received signal and hence is not a serious problem. In some types of satellite-based systems, however, multiple users share the bandwidth simultaneously and the transmissions from any one user occur asynchronously in short bursts, separated by periods of inactivity. In such systems, it is essential that the receiver acquire lock quickly in order to avoid loss of the transmitted data. Satellite oscillator drift, which causes the carrier frequency received from the satellite to vary, can significantly increase signal acquisition time and must therefore be avoided.
Various methods have been employed in an attempt to detect and compensate for frequency drift in satellite-based spread spectrum communication systems. In some systems, for example, a pilot signal is transmitted to the satellite and then received back at a ground station, where the received signal is compared with a local standard to detect drift. In such systems, however, measures must be taken to avoid interference between the pilot signal and the transmitted data. Typically, this is done by transmitting the pilot signal at a carrier frequency which is outside the bandwidth of the information signal. This method, while effective, is disadvantageous in that additional bandwidth is required in order to accommodate the pilot signal. An alternative technique is to provide blanking intervals during which no data is transmitted, so that the receiver is able to detect the phase and frequency of the unmodulated carrier signal. This method avoids the need for an increase in bandwidth, but the net effect is to reduce the amount of data that can be transmitted during a given time interval. Accordingly, a need exists for a system which is capable of detecting and compensating for oscillator drift without requiring additional bandwidth and without affecting the efficiency of data transmission.